Fluid filled vibration isolators are increasingly being utilized to mount engines and transmissions in automobiles. A typical fluid filled isolator may include a pair of superposed variable-volume, fluid filled chambers separated by a partition in which is provided an elongate, generally arcuate inertia track passageway providing continuous fluid communication between the chambers. A so-called decoupler may be provided in the partition between the opposed chambers. The decoupler oscillates in response to alternate pressurization of the fluid in chambers to provide certain dynamic operating characteristics. For example, fluid filled vibration isolators of the aforementioned general type exhibit a certain minimum dynamic stiffness at a relatively low excitation frequency (0-30 Hz.) at large amplitudes (.+-.1.0 mm) and another minimum dynamic stiffness at a higher frequency (above 150 Hz.) under small input amplitude excitations (i.e. .+-.0.1 mm.).
Essentially two types of decouplers are used commercially in fluid filled vibration isolators. One type includes a diaphragm which is either clamped or bonded about its periphery and which flexes in alternate directions as the fluid in the chambers is alternately pressurized. An advantage of diaphragm decouplers is that they are quiet in operation. A disadvantage is that the diaphragm stretches at its periphery during flexure, and such stretching can reduce its fatigue life and hence affect adversely the durability of the vibration isolator. Moreover, it is more difficult for a designer precisely to predict the high frequency dynamic operating characteristics of a vibration isolator having diaphragm-type decouplers.
The other type of decoupler in common use is a relatively stiff disc which oscillates in a translatory manner within a perforate cage located between the fluid chambers. Oscillatory motion is limited by annular cage surfaces which extend across opposite sides of the disc and engage the disc to arrest is translatory motion. An advantage of disc decouplers is that they avoid the fatigue problems associated with diaphragm-type decouplers, and disc decouplers operate in a more precisely predictable manner. A disadvantage is that operation of the decoupler disco under certain operating conditions generates audible noises which can be transmitted into the vehicle. Also disc-type decouplers can present fluid leakage problems.
While both types of decouplers cooperate with their associated inertia track passageways to provide a minimum dynamic stiffness at a relatively high frequency level, it has been difficult for designers to predict precisely the frequency at which such minimum stiffness will occur. Moreover, the magnitude of such minimum stiffness has neither been as low nor as broad in bandwidth as desired for many applications. Also, regardless of the type decoupler employed, a commercially successful fluid filled vibration isolator must be capable of being assembled efficiently.
An effort to achieve some of the foregoing advantages has been made by Metzeler of West Germany. For example, in a commercially available vibration isolator produced by them, a radially ribbed elastomeric decoupler disc (like that illustrated in FIG. 3 herein) is mounted in a cavity in a die cast inertia track decoupler plate assembly. One component of the plate assembly is formed with a shallow cylindrical recess, and the other component includes a flat plate which overlies the recess to provide a decoupler disc cage having parallel opposed surfaces. The decoupler disc fits snugly in the cavity and is exposed to fluid in the opposed chambers via orifices provided in both components of the plate assembly. The inertia track passageway surrounds the decoupler disc cage.
With the Metzeler construction, the maximum excursion of the decoupler disc is quite limited, the cage orifices are relatively small in size, and a joint which can present a leak path between the inertia track passageway and decoupler cavity exists closer to one side of the decoupler disc than the other. While the Metzeler decoupler disc and plate configuration may function satisfactorily and relatively quietly in certain applications, there is a need for a fluid filled vibration isolator that can operate quietly over a broad range of dynamic operating conditions, that can provide a precisely predictable low level minimum dynamic stiffness at a relatively high excitation frequency level, that provides other desirable dynamic operating characteristics at lower frequency levels and at various input excitation amplitudes, and that is leak resistant and capable of being manufactured efficiently by high speed mass production techniques.